问题
I have 4 audio streams in my video file. They are from 4 microphones placed at 4 different people speaking. I want to transcode to a preview file that can be listened to on headphones where everybody's voice can be heard.
I have seen the -ac 2 option, but I can't tell if this will merge all the audio streams or just select the 1st two. I've also seen the amerge filter, but the docs say this will produce 4 audio channels in the output file. So I'm wondering how headphones will deal with the additional 2 channels
回答1:
You have several options. This assumes each individual audio stream in in.mp4 is mono.
Mono
Using the amerge filter and -ac 1:
ffmpeg -i in.mp4 -filter_complex "[0:a]amerge=inputs=4[a]" -ac 1 -map 0:v -map "[a]" -c:v copy out.mp4
With amix:
ffmpeg -i in.mp4 -filter_complex "[0:a]amix=inputs=4[a]" -map 0:v -map "[a]" -c:v copy out.mp4
Stereo
With amerge and -ac 2:
ffmpeg -i in.mp4 -filter_complex "[0:a]amerge=inputs=4[a]" -ac 2 -map 0:v -map "[a]" -c:v copy out.mp4
Manually mixed stereo
Using amerge and pan with custom downmix:
- Channel 0 will be 100% in FL
- Channel 1 will be 75% in FL & 25% in FR
- Channel 2 will be 25% in FL & 75% in FR
- Channel 3 will be 100% in FR
ffmpeg -i in.mp4 -filter_complex "[0:a]amerge=inputs=4,pan=stereo|FL<c0+0.75*c1+0.25*c2|FR<0.25*c1+0.75*c2+c3[a]" -map 0:v -map "[a]" -c:v copy out.mp4
回答2:
Try the amerge audio filter, used to solve this similar question: How do I use ffmpeg to merge all audio streams (in a video file) into one audio channel?
amix should even better should fit your purpose.
来源：https://stackoverflow.com/questions/54060729/ffmpeg-how-to-merge-all-audio-streams-into-stereo